The Renewable Fuels industry is facing increased pressure to expand production, employ a variety of feedstocks particularly those that are available in abundance but may be less desirable or more challenging in use, and to reduce costs. Within the bioethanol industry, there are two main sectors, classified by feedstock, namely: 1) starch and 2) lignocellulosic (biomass). There are approximately 168 operating starch-based ethanol facilities in North America that produced approximately 8 billion gallons of ethanol in 2008. There are approximately 11 biomass-based ethanol plants in the planning stages, with 1 commercial facility currently in operation.
Until recently, the feedstock used for the production of industrial alcohol by fermentation has been sugars from starch (rice, corn, sorghum, etc.), beets, sugar cane or other crops that potentially compete for food. It is believed that these crops will be too expensive in the future to be used as feedstock for the large-scale production of fuel ethanol. The most attractive renewable source of carbohydrates is plant biomass which can be grown in high density, grown on less attractive agricultural land and maintain stable costs. The major fermentable sugars from lignocellulosic materials include glucose, which accounts for about 30-50% of fermentable sugars, and xylose, which accounts for about 10-40% of the fermentable sugars. The most efficient fermentation of lignocellulosic materials would convert both glucose and xylose to ethanol. Unfortunately, to date, microorganisms capable of fermenting both glucose and xylose effectively and efficiently at industrial scale have not been identified.
Saccharomyces yeasts have been used for fermenting glucose-based feedstocks to ethanol and they are considered to be the best microorganisms for that industrial purpose due to their hardiness and tolerance to ethanol, organic acids and adverse environmental conditions. Unfortunately, Saccharomyces yeasts do not metabolize 5-carbon (pentose) sugars, rendering them inefficient for use to ferment plant biomass which comprises significant 5-carbon sugar (e.g. xylose) content. In the last two decades there has been much research into the genetic engineering of yeast and bacteria in order to develop a xylose-fermenting microorganism. However, wild-type and recombinant microorganisms that have been identified as having xylose-metabolizing activity exhibit ineffective xylose production, e.g. either too slow, incomplete, produce too many byproducts or unable to ferment under anaerobic conditions.
The development of recombinant microorganisms to metabolize xylose is hindered by the fact that metabolic pathways involved in xylose metabolism and their regulation, particularly in yeast, are still poorly understood. Although, genetic engineering technology can supply functional xylose metabolizing pathways in yeast that do not normally have this capacity, the regulatory network for xylose fermentation may still not be adequate. Many research facilities are involved in recombinant xylose-metabolizing yeast research yet there is not one successful strain for commercial application to lignocellulosic hydrolysates. Furthermore, there is a lack of knowledge with respect to how recombinant yeast will respond to wood hydrolysate, different feed strategies, oxygenation, inhibitory compounds, etc.
The only commercial demonstration plant producing ethanol from biomass (lignocellulosic material) has reported that they employ a recombinant Saccharomyces strain which was produced by the integration of the genes for xylose reductase (XR) and xylitol dehydrogenase (XDH) from Pichia sp and the Saccharomyces xylulokinase (XK) gene into a polyploid brewing host strain. The resulting strain exhibits conversion of glucose to ethanol, however, the extent, rate and efficiency of xylose conversion to ethanol by this strain is not confirmed.
In view of the foregoing, it is clear that a need exists for a microorganism that can efficiently ferment, e.g. a lignocellulose-based feedstock, to produce ethanol under industrially acceptable conditions.